Gear systems or gear-trains are generally used to transfer rotation and torque between physically separate locations. A gear-train may also permit mechanical advantage to be gained between the input to the gear-train and its output. Gear-trains are typically formed by mounting two or more gears on a frame in a way to permit teeth of the subject gears to engage. For example, gear-trains are frequently employed for transmitting torque from one or more powerplants to power a vehicle.
In order to provide a smooth transmission of rotation in a gear-train from one gear to the next, gear teeth are designed to ensure that the pitch circles of engaging gears roll on each other without slipping. The ratio of the pitch circles of mating gears defines the speed ratio and the mechanical advantage of a specific gear-set. A gear-train may utilize epicyclic or planetary gearing. A gear-train with planetary gearing is capable of providing high gear reduction, i.e., ratio between the input and output, and mechanical advantage in a compact package.